WASHINGTON -- The director of NASA's Marshall Space Flight Center, which is responsible for the foam-shedding external fuel tank that has come under fire in the Columbia disaster investigation, announced Tuesday he is stepping down.

Arthur G. Stephenson, a former president of Houston's Oceaneering Advanced Technologies who has directed the Huntsville, Ala., center since 1998, said his departure is unrelated to Columbia or to any other problems at the space center.

He plans to retire from the space agency in January, and agreed to step down from the director's post early so that NASA can hire his successor in time to help with getting the nation's three remaining shuttles flying again, likely some time next year.

"The people at Marshall and Huntsville are my family, but after five years, I felt it was time to consider new challenges," Stephenson said. "I felt the timing for this move is in the best interest of the agency, Marshall, and me, personally."

Stephenson's boss at NASA headquarters, associate administrator of space flight Bill Readdy, said Stephenson will be reassigned June 15 and will help promote the agency's educational efforts until he retires.

"I worked closely with Art in the office of space flight, and I'm thankful for what he has done for the Marshall Space Flight Center, the people of Alabama, and the entire NASA family," Readdy said.

NASA Administrator Sean O'Keefe praised Stephenson for bringing his private sector management experience to the space agency and called him a "staunch champion of education for our future explorers."

Meanwhile, investigators probing the cause of Columbia's fatal breakup over Texas on Feb. 1 said Tuesday that foam insulation that peeled away from that tank and struck the shuttle's left wing during liftoff may have damaged the spacecraft enough to allow hot gases inside as the seven astronauts headed home.

The Columbia Accident Investigation Board, at its weekly briefing in Houston, also said that film from six previous shuttle flights showed that foam broke off of the tank in the same place as it did during Columbia's liftoff. NASA knew about only four incidents.

Early on in the investigation, Stephenson and others at NASA said engineers eventually became comfortable with the repeated problem of foam falling off the tank during shuttle launches and didn't realize it posed a safety threat.

Marshall also was the space center responsible for the flawed solid rocket booster that doomed space shuttle Challenger, which expoded on liftoff in 1986, killing seven. After that accident, then-Marshall director William Lucas retired.

But NASA News Chief Robert "Doc" Mirelson said Stephenson's departure had nothing to do with Columbia's demise.

Stephenson worked for more than 35 years in the space industry before joining NASA. When he was tapped for the Marshall job by then-NASA Administrator Dan Goldin, he had been supervising Oceaneering's contracts with the space agency and the departments of Defense and Energy.